This invention relates to a terminal fixing structure for a cylindrical ceramic base used in a sensor such as an oxygen sensor.
A general terminal fixing structure for a cylindrical ceramic base with a conductive contact on its surface (e.g., for an oxygen detection element) includes a metal terminal. The terminal has a resilient arm that fits the shape of the ceramic base and engages with the conductive contact on the surface of the ceramic base.
The general terminal fixing structure is used, for example, in an oxygen sensor with a detection element 16 including a pair of porous electrodes 12 and 14 provided on each face of an oxygen ion-conducting solid (e.g., ZrO.sub.2) electrolyte 10, in a test-tube shape, as shown in the partially sectional view of FIG. 6 (Japanese Patent Application No. S61-146210). The oxygen sensor has: a metal member 18; the detection element 16 mounted to the metal member 18 via an insulating ceramic spacer 20, packed talc powders 22, and a ceramic sleeve 24; a protection tube 26 for the detection element 16; a metal terminal 28 connecting to the external electrode 12 of the detection element 16; a metal terminal 32 connecting to the inner electrode 14 of the detection element 16; a metal terminal 33 of an internal ceramic heater 30; leads 34, 36, 38 and 39 respectively connecting to the metal terminals 28, 32, and 33 and to the ceramic heater 30; and housings 40 and 42 for protecting the terminals 28, 32, and so on.
The detection element 16 includes, as shown in FIG. 7, a cylinder 50, a flange 52 engaging with the ceramic spacer 20, and a detection part 54 exposed to the surrounding atmosphere. A conductive contact 56 with the external porous electrode 12, which cannot be seen in FIG. 7 is provided near the opening of the cylinder 50 and is connected to the electrode 12 in the detection part 54 by a lead 58. The electrode 12 in the detection part 54 is covered with a porous protection layer.
The metal terminal 28 for the external electrode includes, as shown in FIG. 7, a pair of curved arms 60 and 62, a holder 64 for holding the lead 34, and a lead 66 for connecting the arms 60 and 62 to the holder 64. The conductive contact 56 of the detection element 16 is held by the resilient arms 60 and 62, thus allowing the metal terminal 28 to be attached to the detection element 16.
The oxygen sensor with this fixing structure is easily manufactured. In this oxygen sensor, an oxygen partial pressure signal generated between the two porous electrodes 12 and 14 of the detection element 16 is outputted from the leads 34 and 36 via the metal terminal 28 for the external electrode and via the metal terminal 32 for the inner electrode, respectively. This oxygen sensor can thus accurately detect the oxygen partial pressure signal without being influenced by noise from an ignition plug or noise caused by a ground voltage, in contrast to another oxygen sensor in which an external porous electrode of a detection element is grounded to an exhaust system via a metal member. An example of the latter oxygen sensor is disclosed in Japanese Patent Publication No. S59-41952.
The terminal fixing structure above, however, has the following problems. Since the terminal 28 is attached to the detection element 16 only by the resilient arms 60 and 62, it might be separated from the element 16 by a vibration of an internal combustion engine, etc, or it might be separated from the element 16 while the oxygen sensor is being assembled if an excessive force applied on the lead 34.